The fabrication of integrated circuits requires a method for accurately forming patterns on a semiconductor wafer. A photoengraving process known as photolithography, or simply masking, is widely employed for this purpose. The microelectronic circuit is built up layer by layer, each layer being based on a pattern received from a photolithographic mask. Such masks typically comprise a glass plate approximately the size of a wafer, the plate having a single pattern repeated many times over its surface. Each repeated pattern corresponds to a pattern to be imposed upon a layer of a wafer.
The mask patterns are derived from an optical reticle having a primary pattern which may be generated by a computer controlled light spot or electron beam which is scanned across a photosensitive plate. The reticle pattern is typically ten times the final size of the pattern to be imposed on the wafer. An image one-tenth the size of the reticle pattern is projected optically on the final mask. The reticle pattern is reproduced side by side many times on the mask, in a step-and-repeat process. Recent advances in reticle production have made it possible to produce reticles having patterns the same size as the final pattern. If such a reticle pattern could be aligned and focused onto a wafer, the mask fabrication could be substantially simplified or entirely eliminated thereby achieving a substantial savings.
The photolithographic process requires that each pattern on the mask be positioned accurately with respect to the layers already formed on the surface of the wafer. One technique is to hold the mask just off the surface of the wafer and to visually align the mask with the patterns in the wafer. After alignment is achieved, the mask is pressed into contact with the wafer. The mask is then flooded with ultraviolet radiation to expose photoresist on the surface of the wafer. The space between the wafer and the mask is often evacuated to achieve intimate contact; atmospheric pressure squeezes the wafer and the mask together. The latter apparatus is typically known as a contact printer. One defect of contact printers is that the masks quickly become abraded and useless. Since mask fabrication is expensive, it would be desirable to have another method that did not wear out the mask.
In view of the foregoing, a recent trend has been toward a technique known as projection alignment, in which an image of the mask pattern is projected onto the wafer through an optical system. In this case, mask life is virtually unlimited. However, one drawback has been that wafer sizes have been increasing, and the task of designing optics capable of projecting an accurate image over the larger area is becoming more difficult. Another drawback is the moveable projection optical system used in some machines for focusing a projected image onto a wafer. It is often difficult to focus such moveable optical systems and to hold the system in focus.
Recent projection aligners have attempted to circumvent the extreme difficulty of constructing a lens capable of resolving micrometer-sized features over an area of many square inches. A much smaller area, on the order of one square centimeter, is exposed, and the exposure is repeated by stepping or scanning the projected image of the mask pattern over the wafer. Such machines are known as projection steppers. So far, all of the efforts to provide commercially acceptable projection steppers have been less than satisfactory. It would be desirable to have a projection stepping machine capable of using the now available, smaller reticles for directly forming patterns on wafers, thereby eliminating the need for a large, multiple pattern mask.